Methods for recovery and recycling of chemicals from sodium sulfite and sodium bisulfite pulping operations

An improved method for the recovery of sodium and sulfite components from the smelt derived from spent sodium sulfite and sodium bisulfite cooking liquor and the preparation of fresh liquor therefrom in which the spent liquor is burned under reduction conditions to a smelt comprised primarily of sodium sulfide and sodium carbonate, the smelt is dissolved in water, the resulting solution clarified, and the clarified solution then processed by the steps of:

• (1) precarbonating the said clarified smelt solution with a gas comprising hydrogen sulfide, carbon dioxide and water vapor to convert the sodium sulfide component to sodium hydrosulfide and sodium carbonate, and a portion of the sodium carbonate component to sodium bicarbonate while absorbing a portion of the carbon dioxide and concentrating the hydrogen sulfide which is then released;
• (2) desulfiding the precarbonated solution by stripping said precarbonated solution with gas containing carbon dioxide and water vapor, to reduce the partial pressure of the hydrogen sulfide which is released from the precarbonated solution during desulfiding and raise the temperature of the precarbonated solution by condensation, while reacting the precarbonated solution with a concentrated sodium bicarbonate solution in which the concentration of sodium bicarbonate is maintained sufficiently high to hold the partial pressure of the carbon dioxide created by the reacted concentrated sodium bicarbonate and precarbonated solutions above the partial pressure of the carbon dioxide provided by the gas utilized for stripping during the stripping operation, thereby preventing carbonation or formation of sodium bicarbonate by carbon dioxide absorption during said stripping, while converting the sodium hydrosulfide in said reacted solution to sodium carbonate, venting said relased hydrogen sulfide and a portion of the carbon dioxide and water vapor used for said stripping for use in step one;
• (3) pressure carbonating at least a portion of the sodium carbonate obtained in step two with carbon dioxide maintained at a pressure above 1.2 atmospheres to convert said portion into a solution with a high concentration of sodium bicarbonate and recycling at least a portion of the high concentration sodium bicarbonate solution to step two;
• (4) decarbonating a portion of the solution from step two containing sodium carbonate and sodium bicarbonte by reacting with sodium bisulfite at a pressure above 1.2 atmospheres to form sodium sulfite and carbon dioxide at pressures above 1.2 atmospheres and recycling the pressurized carbon dioxide to step three;
• (5) reacting the sodium sulfite formed in step four with sulfur dioxide to form sodium bisulfite suitable for use in cooking liquor.